[NF-14] [CONTRACT] Points/Referral Claim and Escrow Contract

Overview

NF-12 and NF-13 establish on-chain referral and points accounting; this issue adds the contract that lets those accrued values actually be claimed/redeemed for something concrete, rather than sitting as a number with no exit path.

Problem

Even with NF-12/NF-13 merged, an accrued points or referral balance is just a read-only number — there is no mechanism to convert it into anything (USDC, bonus shares, discounted fees, etc.), and no escrow holding value to back a future claim.

Proposed Solution

• New contract contracts/rewards_escrow/: holds USDC (or bonus-share-mint authority) funded by Governance-controlled deposits.
• set_redemption_rate(points_per_usdc: i128) — Governance-gated.
• claim(user: Address) -> i128 — reads the user's current points balance from the points contract (NF-13) and referral-bonus-owed from the referral accounting (NF-12), computes the redeemable USDC amount at the current rate, zeroes out the claimed balance in the source contract(s), and transfers USDC to the user.
• Must be safe against double-claiming (zero the balance atomically as part of the same transaction as the transfer).

Acceptance Criteria

• claim correctly computes redeemable amount from current points + referral balances and the governance-set rate
• A user cannot claim the same points twice
• Escrow running low on funds fails claims gracefully with a clear error rather than a panic that could corrupt accounting
• Redemption rate is Governance-gated
• Unit tests cover: successful claim, double-claim rejection, insufficient-escrow-balance handling

Start by reading the existing NF-12 referral accounting and NF-13 points contract interfaces, then inspect the proposed contracts/rewards_escrow/ entry point. Trace how balances, governance authorization, and value transfers are represented before defining the cross-contract claim flow. Done means the acceptance tests pass for redemption, double-claim prevention, insufficient escrow funds, and governance-gated rates.